The ophthalmoscope is well known as an important aid for studying and examining the eye, and in particular, the fundus of the eye. As a result of great interest in preserving man's eyesight, ophthalmoscopes of various constructions have been built and used. Early ophthalmoscopes were mechanically manipulated so that, with an illumination source directed at the eye, the observer--using an optical system similar to a microscope--observed the fundus by manually moving the optical viewing system relative to the axis of the eye. Various constructions of more sophisticated opthalmoscopes have evolved which provide the ophthalmologist and laboratory researcher with a fine manually operated instrument for observing the retina and other layers of the fundus.
Concurrently, starting in about 1950, a number of ophthalmoscopes incorporating a television system were proposed. These "active" systems generally used the optical system of a conventional fundus camera, having a relatively small field of view, to display an image on a television screen. These systems, however, have generally been unsatisfactory because they need a large and often uncomfortable amount of illumination to overcome the low reflectivity of the fundus. Also, the systems are subject to image deterioration by high levels of stray light and scatter. The stray light appears as noise or "fog" and has caused the systems, which usually use commercial television camera components, to have low resolution and contrast. These systems do not provide the higher quality photographic images of conventional, commercially available fundus cameras, which do not need as high a light level and which provide a high resolution photograph. Thus, the "television ophthalmoscope" has not, thus far, seriously challenged either visual observation or photographic recording of the fundus.
Nevertheless, ophthalmoscopes incorporating a television system have been built. These units generally use a Zeiss or Topcon fundus camera optics for creating a first image of the fundus for TV viewing, and provide a field of view at the fundus of about 9 millimeters in diameter (or 30.degree. of the retina). Thus, the TV camera's effective picture element, that is, the width of one picture line, extends over about 17 micrometers, i.e. 9 mm divided by 525 lines of a standard television scan, of the retina. The commercial systems are, however, subject to the above-noted significant light scattering.
As an alternative to the television systems, it has been proposed to use a cathode-ray-tube flying-spot scanning system in an ophthalmoscope, so that the illumination source is a well-defined illuminating beam. The illumination source for this device would thus be a point of light which moves across or scans the subject. A single photodetector is used to collect the reflected light. In such a system, only a single location or spot of the fundus is illuminated at any particular time, and the detector signal is derived solely from reflections due to illumination of that spot. Therefore, so long as the visual display is synchronized with the movement of the scanning spot, the visual display provides a representation of the scanned area of the fundus.
Flying spot systems, however, have generally been "light starved". That is, cathode ray tubes used as the illumination source generally lack the amount of light required to provide a good signal to noise ratio. This occurs even though the systems scan only a small field of view, for example, that photographed by the Zeiss camera. These systems, therefore, suffer from a poor signal to noise ratio because of the low contrast that results from low illumination in combination with light scatter.
Furthermore, the television systems as well as prior flying spot scanning systems have thus far not been adaptable to color imaging, because generating a color image requires more light than monochromatic imaging. Nor have they been adaptable for use with fluorescein angiography.
Principal objects of this invention are therefore to provide a scanning ophthalmoscope having improved resolution, having improved contrast, which uses a relatively low average light level, which has an improved depth of field and focus, and which can provide color imaging at high resolution.
Other objects of the invention are to provide a scanning opthalmoscope wherein image quality is independent of the regularity of the fundus structure, which operates with relatively low noise, which is safe for the subject, which is reliable, and which provides the user with great operational flexibility.